Among currently employed processes for synthesizing acetic acid, one of the most used commercially is the catalyzed carbonylation of methanol with carbon monoxide. Preferred methods of practicing this technology include so-called “low water” processes catalyzed with rhodium or iridium of the class seen in commonly assigned U.S. Pat. No. 5,001,259, issued Mar. 19, 1991; U.S. Pat. No. 5,026,908, issued Jun. 25, 1991; and U.S. Pat. No. 5,144,068, issued Sep. 1, 1992; as well as European Patent No. EP 0 161 874 B2, published Jul. 1, 1992. The features involved in practicing a low water carbonylation process may include maintaining in the reaction medium, along with a catalytically effective amount of rhodium and at least a finite concentration of water, an elevated concentration of inorganic iodide anion over and above the iodide ion that is present due to hydrogen iodide in the system. This iodide ion may be a simple salt, with lithium iodide being preferred in most cases. U.S. Pat. Nos. 5,001,259, 5,026,908, 5,144,068 and European Patent No. EP 0 161 874 B2 are herein incorporated by reference.
It has been found that although a low water carbonylation process for producing acetic acid increases carbon monoxide efficiency by reducing by-products carbon dioxide and hydrogen, the amount of other impurities, such as acetaldehyde and its derivatives, increase in a low water carbonylation process as opposed to similar processes operated with higher water concentrations in the reactor. These impurities affect quality of acetic acid, especially when they are recirculated through the reaction process which causes build up of derivative impurities, such as carbonyl compounds and alkyl iodides. Carbonyl impurities decrease the permanganate time of the acetic acid, a quality test commonly used in the industry. As used herein, the phrase “carbonyl” is intended to mean compounds that contain aldehyde or ketone functional groups, which compounds may or may not possess unsaturation. See Catalysis of Organic Reaction, 75, 369-380 (1998), for further discussion on impurities in a carbonylation process.
The present invention is directed, in part, to reducing and/or removing permanganate reducing compounds (PRC's) such as acetaldehyde, acetone, methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde and the like, alkyl iodides that may be derived from acetaldehyde and the aldol condensation products of the impurities. The present invention may also lead to reduction of propionic acid formation in some cases because acetaldehyde formation appears to cause increases in propionic acid levels, perhaps because of the availability of hydrogen in the reactor. Without intending to be bound by theory, it is believed that many impurities are derived from acetaldehyde which appears to form more readily in the presence of iodide salts, for example, lithium iodide. Acetaldehyde condenses to form unsaturated aldehydes, such as crotonaldehyde, which may then generate higher alkyl iodides in the system which are particularly difficult to remove and are poisonous to vinyl acetate catalysts. Vinyl acetate production is the single largest end-use of acetic acid.
Conventional techniques to remove acetaldehyde impurities include treating the acetic acid product streams with low concentrations of aldehyde impurity with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like. Such treatments may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the final product. It is also known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine, which reacts with the carbonyl compounds to form oximes, followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the final product adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.
Other processes have been described for producing high purity acetic acid by distilling light ends condensed from the light ends stripper column to remove acetaldehyde. Streams suggested for processing to remove acetaldehyde include a light phase containing primarily water, acetic acid and methyl acetate; or a heavy phase containing primarily methyl iodide, methyl acetate and acetic acid; or a stream formed by combining the light and heavy phase. It has been disclosed, for example, in commonly assigned U.S. Pat. Nos. 6,143,930 and 6,339,171, that it is possible to significantly reduce the undesirable impurities in the acetic acid product by performing a multi-stage purification on the condensed light ends column overhead. These patents disclose a purification process in which the light ends overhead is distilled twice, in each case taking the acetaldehyde overhead and returning a methyl iodide rich residuum to the reactor. The acetaldehyde-rich distillate obtained after the two distillation steps is optionally extracted with water to remove the majority of the acetaldehyde for disposal, leaving a significantly lower acetaldehyde concentration in the raffinate that is recycled to the reactor. U.S. Pat. Nos. 6,143,930 and 6,339,171 are incorporated herein by reference in their entirety. Additional systems for removing aldehydes and other permanganate reducing compounds are described in U.S. patent application Ser. No. 11/116,771 (Publication No. US 2006/0247466 A1) of Zinobile et al., entitled “Process for the Production of Acetic Acid”; U.S. patent application Ser. No. 10/708,420 (Publication No. US 2005/0197508 A1) of Scates et al., entitled “Removal of Permanganate Reducing Compounds from Methanol Carbonylation Process Stream”; and U.S. patent application Ser. No. 10/708,421 (Publication No. US 2005/0197509 A1) of Picard et al., entitled “Removal of Permanganate Reducing Compounds from Methanol Carbonylation Process Stream”; the disclosures of which are hereby incorporated by reference. In general, the condensed light ends contain less than 1 weight percent acetaldehyde.
While the above-described processes have been successful in removing carbonyl impurities from the carbonylation system and controlling acetaldehyde levels and permanganate time, existing procedures tend to be expensive in terms of both capital and operating costs due to the low levels of aldehyde impurity that need to be removed from a particular stream to prevent build-up of aldehyde and related impurities in the system. Accordingly, there remains a need for alternative processes to improve the efficiency of acetaldehyde removal in methanol carbonylation processes.